Field of the Invention
The invention relates to a method for the determination of a reserve torque in an electronically-switched drive mechanism, specifically a pulse motor for a headlamp beam adjustment system, based upon a parameter in an essentially linear relationship to a back electromotive force of the drive mechanism.
Electronically-switched drive mechanisms, also described as “Brushless DC Motors” or “BLDCs”, in various forms of embodiment are known from the prior art, whereby drive mechanisms of this type are used in the automobile industry for various applications, for example in headlamp beam adjustment systems, e.g. as pulse motors.
A pulse motor of this type is described in published, non-prosecuted German patent application DE 10 2012 104 541 A1, corresponding to U.S. patent publication No. 2012/0304914. The pulse motor contains a first conductive core, the ends of which are gapped in relation to each other, and a second conductive core, the ends of which are also mutually gapped. The conductive cores are generally configured in a C shape, and arranged at right-angles to each other, such that the first core ends are adjacent to the second core ends. The conductive cores are both comprised of a magnetic material, for the conduction of a magnetic field. The pulse motor also contains a permanent magnet, which is arranged between the core ends. The permanent magnet is mounted on a rotatable rotor shaft. In addition, a first induction coil is wound around at least one part of the first conductive core, and a second induction coil is wound around at least one part of the second conductive core. The first induction coil is provided with a first set of supply conductors, and the second induction coil is provided with a second set of supply conductors. The first and second supply conductors are electrically connected to a programmable control system, which is electrically connected to a power source for the supply of electric current to the first and second induction coils. The first and second conductive cores may be magnetized accordingly. In service, the control circuit selectively delivers a voltage to the first and second supply conductors, such that an electric current flows in the first and second induction coils respectively. In case of a change in the electric current, a magnetic field is induced in the associated induction coil. The magnetic field is then channeled through the connected conductive core towards the permanent magnet. If the magnetic field of the magnetized conductive core and the magnetic field of the permanent magnet are not in alignment, the permanent magnet will rotate around the axis of the rotor shaft. In order to maintain the rotation of the permanent magnet, the magnetic field of the first and second induction coils is altered accordingly by the application of a sequence of current signals. The speed of rotation can then be controlled by adjusting the magnitude of the voltage or current applied to the first and second supply conductors, and by the synchronized adjustment of the voltages or current.
A method for the detection of a stalled state of the pulse motor is also known, e.g. from published, non-prosecuted German patent application DE 10 2012 104 541 A1, wherein the “back electromotive force” (or “back-EMF”) of the induction coils is measured. Where the control system excites one of the induction coils, and the other induction coil is de-excited, the rotation of the permanent magnet induces a voltage in the de-excited coil. This voltage is the back-EMF, and can be measured by the control system. A substantial reading for back-EMF indicates that the permanent magnet is rotating and, accordingly, that the pulse motor is not in the stalled state. Conversely, a low reading for back-EMF indicates that the permanent magnet is stationary and, accordingly, that the pulse motor is in the stalled state. In a method applied to determine whether the pulse motor is in the stalled state, the pulse motor is rotated in one direction until it reaches a limit stop, during which time the back-EMF is monitored by the control system. Immediately the back-EMF falls below a predetermined threshold, the control system rotates the pulse motor back in the other direction, while again monitoring the back-EMF, until a second limit stop is reached, at which the back-EMF again falls below a predetermined threshold. This reference procedure is generally described as a “reference run”.
This method is specifically disadvantageous, in that the change in the back-EMF in the critical range is small, such that the known method only permits the achievement of maximum torque to be evaluated with limited accuracy. It is also disadvantageous that the threshold applied for stall detection is dependent upon the load. It has also been observed that, as a result of vibrations in the motor, the back-EMF may remain above the threshold, such that the stalled state cannot be reliably anticipated.
In the automobile industry, pulse motors of the type described above are used e.g. in headlight beam adjustment systems. It may be observed that the luminous intensity of available headlights increases from generation to generation.
Accordingly, it is necessary to ensure that headlights are not left in a position in which oncoming traffic—the density of which continues to rise as vehicle numbers increase—will be blinded, startled and/or dazzled. The blinding of oncoming traffic increases accident risk.
In order to resolve this problem, the prior art has already described how, in generic drive mechanisms, the point of maximum back-EMF relative to the maximum drive current indicates the reserve torque of the drive mechanism. By this method, it is possible to conduct a diagnosis of the mechanical system of the drive mechanism, and any points of sluggishness can be gauged. To this end, torque characteristic curves can be recorded, the movement of which over time constitutes a measure of wear in the mechanical system.
By the early detection of signs of wear in the motor or in the headlight, headlights can be switched to a safe condition prior to the complete loss of function, thereby reducing the dazzling of oncoming traffic and minimizing the accident risk. In other applications, it is also necessary to determine the reserve torque of the electrically-switched drive mechanism as accurately as possible.
To this end, the prior art provides for the determination of the back-EMF at the zero-crossing of the drive current, at the point where the voltage drop associated with ohmic resistance disappears. This arrangement should ensure that the measurement of the back-EMF does not impair the control of the motor.
Disadvantageously, however, this method has proven to be of limited robustness. Moreover, the measurement of the back-EMF at the zero-crossing of the drive current has the disadvantage that this method cannot normally determine the amplitude of the back-EMF, as this variable coincides with no-load conditions, but not with on-load conditions at the zero-crossing of the drive current. Consequently, a determination of the reserve torque with greater accuracy is not possible.